U.S. patent application number 10/736542 was filed with the patent office on 2004-07-01 for radio communication apparatus, radio communication system, and communication control method.
This patent application is currently assigned to MITSUBISHI MATERIALS CORPORATION. Invention is credited to Ishikawa, Mototaka, Nakamura, Kenzo, Tari, Kazuyoshi.
Application Number | 20040125894 10/736542 |
Document ID | / |
Family ID | 18129451 |
Filed Date | 2004-07-01 |
United States Patent
Application |
20040125894 |
Kind Code |
A1 |
Nakamura, Kenzo ; et
al. |
July 1, 2004 |
Radio communication apparatus, radio communication system, and
communication control method
Abstract
A radio communication apparatus which uses one of phase shift
keying (PSK), differential phase shift keying (DPSK), or quadrature
amplitude modulation (QAM), and in which frequency synchronization
is achieved based on calculated phase difference data associated
with a received signal at time intervals shorter than symbol
intervals. The radio communication apparatus includes a frequency
synchronization control unit for correcting a frequency of a
reference signal used to achieve frequency synchronization in
accordance with a mean value of the phase difference data
calculated over a period of time in which two or more symbols are
input. The radio communication apparatus also includes a phase
compensation control unit for adaptively controlling a phase of the
received signal when the frequency synchronization control unit
achieves frequency synchronization such that an error of the
frequency of the reference signal relative to a frequency of the
received signal has fallen within a predetermined range, thereby
ensuring that frequency synchronization is achieved even when a
phase rotation greater than .pi./4 per cycle occurs.
Inventors: |
Nakamura, Kenzo; (Omiya-shi,
JP) ; Tari, Kazuyoshi; (Omiya-shi, JP) ;
Ishikawa, Mototaka; (Omiya-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
MITSUBISHI MATERIALS
CORPORATION
Tokyo
JP
|
Family ID: |
18129451 |
Appl. No.: |
10/736542 |
Filed: |
December 17, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10736542 |
Dec 17, 2003 |
|
|
|
09439097 |
Nov 12, 1999 |
|
|
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Current U.S.
Class: |
375/326 ;
375/327 |
Current CPC
Class: |
H04L 2027/0065 20130101;
H04L 2027/0042 20130101; H04L 27/227 20130101 |
Class at
Publication: |
375/326 ;
375/327 |
International
Class: |
H04L 027/14; H04L
027/16 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 11, 1998 |
JP |
10-321157 |
Claims
1. A radio communication apparatus which uses one of phase shift
keying (PSK), differential phase shift keying (DPSK), or quadrature
amplitude modulation (QAM), and in which frequency synchronization
is achieved based on calculated phase difference data associated
with a received signal at time intervals shorter than symbol
intervals, the radio communication apparatus including: frequency
synchronization control means for correcting a frequency of a
reference signal used to achieve frequency synchronization in
accordance with a mean value of the phase difference data
calculated over a period of time in which two or more symbols are
input.
2. A radio communication apparatus according to claim 1, further
comprising: phase compensation control means for adaptively
controlling a phase of the received signal when the frequency
synchronization control means achieves frequency synchronization
such that an error of the frequency of the reference signal
relative to a frequency of the received signal has fallen within a
predetermined range.
3. A radio communication apparatus according to claim 2, wherein
the frequency synchronization control means adaptively increases or
decreases a sampling number and a sampling time of the calculated
phase difference data subjected to the mean value calculation, in
accordance with the error of the frequency of the reference signal
relative to the frequency of the received signal.
4. A radio communication apparatus according to claim 2, further
including: frequency error evaluation means for determining whether
the error of the frequency of the reference signal relative to the
frequency of the received signal has fallen within the
predetermined range after the frequency synchronization control
means corrects the frequency of the reference signal; and means for
starting communication to a sender based on the frequency within
the predetermined range, when the frequency error evaluation means
determines that the error is within the predetermined range.
5. A radio communication apparatus according to claim 2, further
comprising: means for correcting, by a correction value
corresponding to a value of phase compensation made via adaptive
phase control by the phase compensation control means, control data
used to control the frequency of the reference signal and which
corresponds to a frequency correction value required to achieve
frequency synchronization by the frequency synchronization control
means; means for setting the frequency of the reference signal in
accordance with the control data corrected by the correcting means;
and means for starting communication to a sender.
6. A radio communication apparatus according to claim 4, further
comprising: means for performing a receiving operation under
conditions in which the frequency error evaluation means determines
the error is within the predetermined range, wherein when the
receiving operation is again started after a pause, the frequency
synchronization control means performs frequency synchronization
control using the frequency of the reference signal at that time
such that the error of the frequency of the received signal
relative to the reference signal falls within the predetermined
range, and when the receiving operation is again started after
another pause, the receiving operation is performed using the same
frequency of the reference signal as the frequency used in the
previous receiving operation.
7. A radio communication apparatus according to claim 2, wherein
the phase compensation control means comprises a fractionally
spaced equalizer.
8. A radio communication apparatus according to claim 2, further
comprising: means for setting a response speed of frequency
correction control performed by the frequency synchronization
control means to be lower than a response speed of the adaptive
phase control performed by the phase compensation control
means.
9. A radio communication apparatus which uses one of phase shift
keying (PSK), differential phase shift keying (DPSK), or quadrature
amplitude modulation (QAM), and in which frequency synchronization
is achieved based on calculated phase difference data associated
with a received signal at time intervals shorter than symbol
intervals, the radio communication apparatus including: a frequency
synchronization control unit configured to correct a frequency of a
reference signal used to achieve frequency synchronization in
accordance with a mean value of the phase difference data
calculated over a period of time in which two or more symbols are
input.
10. A radio communication apparatus according to claim 9, further
comprising: a phase compensation control unit configured to
adaptively control a phase of the received signal when the
frequency synchronization control unit achieves frequency
synchronization such that an error of the frequency of the
reference signal relative to a frequency of the received signal has
fallen within a predetermined range.
11. A radio communication apparatus according to claim 10, wherein
the frequency synchronization control unit adaptively increases or
decreases a sampling number and a sampling time of the calculated
phase difference data subjected to the mean value calculation, in
accordance with the error of the frequency of the reference signal
relative to the frequency of the received signal.
12. A radio communication apparatus according to claim 10, further
including: a frequency error evaluator configured to determine
whether the error of the frequency of the reference signal relative
to the frequency of the received signal has fallen within the
predetermined range after the frequency synchronization control
unit corrects the frequency of the reference signal, wherein
communication to a sender is started based on the frequency within
the predetermined range, when the frequency error evaluator
determines that the error is within the predetermined range.
13. A radio communication apparatus according to claim 10, further
comprising: a correcting mechanism configured to correct, by a
correction value corresponding to a value of phase compensation
made via adaptive phase control by the phase compensation control
unit, control data used to control the frequency of the reference
signal and which corresponds to a frequency correction value
required to achieve frequency synchronization by the frequency
synchronization control unit; a setting mechanism configured to set
the frequency of the reference signal in accordance with the
control data corrected by the correcting mechanism; and a starting
mechanism configured to start communication to a sender.
14. A radio communication apparatus according to claim 12, wherein
a receiving operation is performed under conditions in which the
frequency error evaluator determines the error is within the
predetermined range, when the receiving operation is again started
after a pause, the frequency synchronization control unit performs
frequency synchronization control using the frequency of the
reference signal at that time such that the error of the frequency
of the received signal relative to the reference signal falls
within the predetermined range, and when the receiving operation is
again started after another pause, the receiving operation is
performed using the same frequency of the reference signal as the
frequency used in the previous receiving operation.
15. A radio communication apparatus according to claim 2, wherein
the phase compensation control unit comprises a fractionally spaced
equalizer.
16. A radio communication apparatus according to claim 2, further
comprising: a setting mechanism configured to set a response speed
of frequency correction control performed by the frequency
synchronization control unit to be lower than a response speed of
the adaptive phase control performed by the phase compensation
control unit.
17. A radio communication method which uses one of phase shift
keying (PSK), differential phase shift keying (DPSK), or quadrature
amplitude modulation (QAM), and in which frequency synchronization
is achieved based on calculated phase difference data associated
with a received signal at time intervals shorter than symbol
intervals, the radio communication method comprising the steps of:
correcting a frequency of a reference signal used to achieve
frequency synchronization in accordance with a mean value of the
phase difference data calculated over a period of time in which two
or more symbols are input.
18. A radio communication method according to claim 17, further
comprising the step of: adaptively controlling a phase of the
received signal when the correcting step achieves frequency
synchronization such that an error of the frequency of the
reference signal relative to a frequency of the received signal has
fallen within a predetermined range.
19. A radio communication method according to claim 18, wherein the
correcting step adaptively increases or decreases a sampling number
and a sampling time of the calculated phase difference data
subjected to the mean value calculation, in accordance with the
error of the frequency of the reference signal relative to the
frequency of the received signal.
20. A radio communication method according to claim 18, further
comprising the steps of: determining whether the error of the
frequency of the reference signal relative to the frequency of the
received signal has fallen within the predetermined range after the
correcting step corrects the frequency of the reference signal; and
starting communication to a sender based on the frequency within
the predetermined range, when the determining step determines that
the error is within the predetermined range.
21. A radio communication method according to claim 18, further
comprising the steps of: correcting, by a correction value
corresponding to a value of phase compensation made via adaptive
phase control by the adaptively controlling step, control data used
to control the frequency of the reference signal and which
corresponds to a frequency correction value required to achieve
frequency synchronization; setting the frequency of the reference
signal in accordance with the corrected control data; and starting
communication to a sender.
22. A radio communication method according to claim 20, further
comprising the steps of: performing a receiving operation under
conditions in which the determining step determines the error is
within the predetermined range, wherein when the receiving
operation is again started after a pause, the correcting step
performs frequency synchronization control using the frequency of
the reference signal at that time such that the error of the
frequency of the received signal relative to the reference signal
falls within the predetermined range, and when the receiving
operation is again started after another pause, the receiving
operation is performed using the same frequency of the reference
signal as the frequency used in the previous receiving
operation.
23. A radio communication method according to claim 18, further
comprising the step of: setting a response speed of frequency
correction control performed by the correcting step to be lower
than a response speed of the adaptive phase control performed by
the adaptively controlling step.
24. A computer program product for executing a radio communication
method which uses one of phase shift keying (PSK), differential
phase shift keying (DPSK), or quadrature amplitude modulation
(QAM), and in which frequency synchronization is achieved based on
calculated phase difference data associated with a received signal
at time intervals shorter than symbol intervals, the computer
program product comprising: a first computer code configured to
correct a frequency of a reference signal used to achieve frequency
synchronization in accordance with a mean value of the phase
difference data calculated over a period of time in which two or
more symbols are input.
25. The computer program product according to claim 24, further
comprising: a second computer code configured to adaptively control
a phase of the received signal when the first computer code
achieves frequency synchronization such that an error of the
frequency of the reference signal relative to a frequency of the
received signal has fallen within a predetermined range.
26. The computer program product according to claim 25, wherein the
first computer code adaptively increases or decreases a sampling
number and a sampling time of the calculated phase difference data
subjected to the mean value calculation, in accordance with the
error of the frequency of the reference signal relative to the
frequency of the received signal.
27. The computer program product according to claim 25, further
comprising: a third computer code configured to determine whether
the error of the frequency of the reference signal relative to the
frequency of the received signal has fallen within the
predetermined range after the first computer code corrects the
frequency of the reference signal; and a fourth computer code
configured to start communication to a sender based on the
frequency within the predetermined range, when the third computer
code determines that the error is within the predetermined
range.
28. The computer program product according to claim 25, further
comprising: a third computer code configured to correct, by a
correction value corresponding to a value of phase compensation
made via adaptive phase control by the second computer code,
control data used to control the frequency of the reference signal
and which corresponds to a frequency correction value required to
achieve frequency synchronization; a fourth computer code
configured to set the frequency of the reference signal in
accordance with the control data corrected by the third computer
code; and a fifth computer code configured to start communication
to a sender.
29. The computer program product according to claim 27, further
comprising: a fifth computer code configured to perform a receiving
operation under conditions in which the third computer code
determines the error is within the predetermined range, wherein
when the receiving operation is again started after a pause, the
first computer code performs frequency synchronization control
using the frequency of the reference signal at that time such that
the error of the frequency of the received signal relative to the
reference signal falls within the predetermined range, and when the
receiving operation is again started after another pause, the
receiving operation is performed using the same frequency of the
reference signal as the frequency used in the previous receiving
operation.
30. The computer program product according to claim 25, further
comprising: a third computer code configured to set a response
speed of the frequency correction control performed by the first
computer code to be lower than a response speed of the adaptive
phase control performed by the second computer code.
31. A radio communication system employing a two-frequency simplex
method in which communications between a base station and a
plurality of mobile stations are performed in such a manner that
the base station continuously transmits a signal whereas the mobile
stations transmit a bust signal, wherein at least one of the base
station and the plurality of mobile stations, comprises: a
frequency synchronization control unit configured to correct a
frequency of a reference signal used to achieve frequency
synchronization in accordance with a mean value of the phase
difference data calculated over a period of time in which two or
more symbols are input.
32. The system according to claim 31, wherein the at least one of
the base station and the plurality of mobile stations further
comprises: a phrase compensation control unit configured to
adaptively control a phase of the received signal when the
frequency synchronization control unit achieves frequency
synchronization such that an error of the frequency of the
reference signal relative to a frequency of the received signal has
fallen within a predetermined range.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to Japanese Application No.
Hei 10-321157, filed on Nov. 11, 1998, which is incorporated in its
entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a radio communication
apparatus, a radio communication system using phase shift keying
(PSK), differential phase shift keying (DPSK), or quadrature
amplitude modulation (QAM), in which frequency synchronization is
accomplished on the basis of phase difference data, at time
intervals shorter than symbol intervals, associated with a received
signal. The present invention also relates to a radio communication
system including such a radio communication apparatus, and to a
communication control method.
[0004] 2. Discussion of the Background
[0005] In the art of digital communication using digital modulation
such as quadrature amplitude modulation and phase shift keying,
phase locking is performed using, for example, the costas method.
In digital modulation, such as QPSK, the phase becomes equal to
.pi./4 at fixed intervals. This property is used to correct the
phase error as follows. That is, the value 4 times the angle is
determined at the fixed intervals and the phase error is corrected
on the basis of the obtained value. However, this causes an
instability of .pi./2. Furthermore, if a phase error greater than
.pi./4 occurs in one period, it is impossible to correct the
frequency error.
[0006] In narrow-band digital communication systems, a low symbol
rate is employed, and thus a frequency offset is a great problem.
Therefore, when phase locking is performed using the costas method,
a high-stability oscillator dedicated to frequency synchronization
is required to achieve a small phase error less than .pi./4 per
symbol interval. However, the employment of such a high-stability
oscillator causes an increase in cost.
[0007] To quickly achieve frequency synchronization, adaptive phase
control using a linear prediction method or the like is
conventionally employed. Although this technique is capable of
quickly achieving frequency synchronization, a problem is that a
training signal is required. Furthermore, as in the costas method,
it becomes impossible to achieve synchronization if a phase error
greater than .pi./4 occurs in one period.
[0008] Japanese Patent No. 2743826 discloses a radio communication
system in which frequency synchronization between primary and
secondary stations is achieved as follows. A secondary station
reproduces a reference clock signal from a signal received from a
primary station and detects a difference in frequency between the
reproduced reference clock signal and a reference carrier signal
(reference clock) used to transmit a burst signal. The frequency of
the reference carrier signal is controlled so that the frequency
difference becomes constant, thereby achieving frequency
synchronization.
[0009] This radio communication system needs a frequency counter
for detecting the frequency error. Furthermore, if the frequency of
the reference carrier used to transmit a signal is tried to be
locked with the reference clock signal extracted from the received
signal so as to achieve high-precision synchronization, phase
jitter occurs. A phase variation due to modulation is another
problem when frequency synchronization is accomplished using the
reference clock signal extracted from the received signal. This
problem is serious particularly in narrow-band communications.
[0010] Japanese Unexamined Patent Publication Nos. 5-75662 and
6-326740 disclose phase locking techniques. However, in these phase
locking techniques, a problem also occurs when there is a phase
error greater than .pi./4, and the problem of phase jitter is not
solved in these techniques.
[0011] Japanese Unexamined Patent Publication Nos. 6-318963 and
6-197140 disclose techniques of handling phase errors greater than
.pi./4 by arbitrarily offsetting the frequency. However, the
offsetting of the frequency can cause an error or instability in
frequency synchronization when the occurrence of some particular
data is great compared with other data.
[0012] Japanese Unexamined Patent Publication No. 6-261089
discloses a technique of correcting a phase error greater than
.pi./4 by reducing the sampling interval. In this technique, a
phase difference is determined within one symbol period, and the
phase error is determined from the phase difference and the phase
rotation direction (polarity). However, phase jitter can cause a
problem when the phase difference is determined within one symbol
period. Another problem of this technique is ambiguity which occurs
when a phase rotation greater than .pi./4 occurs. Furthermore, when
the roll-off factor is small, a problem can occur if the phase
error is determined from the polarity of a signal passed through a
bandpass filter.
SUMMARY OF THE INVENTION
[0013] Accordingly, one object of the present invention is to solve
the above-noted and other problems.
[0014] Another object of the present invention is to provide a
radio communication apparatus, a radio communication system, and a
communication control method, in which frequency synchronization
and high-precision phase compensation is achieved, even when a
phase rotation greater than .pi./4 occurs in one period.
[0015] To achieve these and other objects, the present invention
provides a radio communication apparatus which uses one of phase
shift keying (PSK), differential phase shift keying (DPSK), or
quadrature amplitude modulation (QAM), and in which frequency
synchronization is achieved based on calculated phase difference
data associated with a received signal at time intervals shorter
than symbol intervals. The radio communication apparatus includes a
frequency synchronization control unit for correcting a frequency
of a reference signal used to achieve frequency synchronization in
accordance with a mean value of the phase difference data
calculated over a period of time in which two or more symbols are
input. The radio communication apparatus also includes a phase
compensation control unit for adaptively controlling a phase of the
received signal when the frequency synchronization control unit
achieves frequency synchronization such that an error of the
frequency of the reference signal relative to a frequency of the
received signal has fallen within a predetermined range.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] Various other objects, features and attendant advantages of
the present invention will be more fully appreciated as the same
becomes better understood from the following detailed description
when considered in connection with the accompanying drawings in
which like reference characters designate like or corresponding
parts throughout the several views and wherein:
[0017] FIG. 1 is a block diagram illustrating an embodiment of a
radio communication apparatus according to the present
invention;
[0018] FIG. 2 is a flow chart illustrating a control operation of a
frequency synchronization control unit shown in FIG. 1;
[0019] FIG. 3 is a flow chart illustrating a control operation of a
frequency synchronization control unit shown in FIG. 1;
[0020] FIG. 4 is a flow chart illustrating a control operation of a
phase compensation control unit shown in FIG. 1;
[0021] FIG. 5 is a schematic representation of an adaptive phase
control operation;
[0022] FIG. 6 is a graph illustrating an example of the adaptive
phase control characteristic;
[0023] FIG. 7 is a block diagram illustrating main parts of another
embodiment of a radio communication apparatus according to the
present invention; and
[0024] FIG. 8 is a schematic view illustrating a radio
communication system according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] Referring now to the drawings, wherein like reference
numerals designate identical or corresponding parts throughout the
several views, FIG. 1 illustrates a radio communication apparatus
according to a first embodiment of the present invention. As shown
in FIG. 1, the radio communication apparatus includes an antenna
10, a voltage controlled oscillator (VCO) 12 for generating a
carrier reference signal used to convert a QPSK (quadrature phase
shift keying) signal received via the antenna 10 into a baseband
signal, and a multiplier 14. Also included is an analog-to-digital
converter 16 for converting the output of the multiplier 14 from
analog form into digital form, and a quadrature demodulator 18 for
converting, by quadrature demodulation, the baseband signal into an
in-phase component (I) and a quadrature component (Q).
[0026] The radio communication further includes an over-sampling
phase difference detector 22, which over-samples the output of the
quadrature demodulator and detects phase difference data at time
intervals shorter than symbol intervals over a period of time in
which two or more symbols are input. Also included is a phase
difference averaging unit 24 for calculating a mean value of the
phase difference data output from the over-sampling phase
difference detector 22, a frequency-to-voltage converter 26 for
converting into a voltage form the frequency error data which is
output from the phase difference averaging unit 24 and which
indicates an error of the frequency of the reference carrier signal
output from the voltage controlled oscillator 12 relative to the
received signal, and a digital-to-analog converter 28 for
converting the output signal of the frequency-to-voltage converter
26 from digital form into analog form.
[0027] As shown, a frequency synchronization control unit 20
includes the over-sampling phase difference detector 22, the phase
difference averaging unit 24, the frequency-to-voltage converter
26, and the digital-to-analog converter 28. Using the mean value of
the phase difference data associated with the received signal,
calculated over a period of time in which two or more symbols are
input, the frequency synchronization control unit 20 corrects the
frequency of the output signal of the voltage controlled oscillator
12 serving as the reference signal used to achieve frequency
synchronization with the received signal, such that the frequency
error of the reference signal relative to the received signal
becomes smaller than a predetermined maximum allowable frequency
error.
[0028] The radio communication apparatus according to the first
embodiment further includes a phase compensation control unit 30
for precisely controlling a phase compensation of the demodulated
signal output from the quadrature demodulator 18. The phase
compensation control unit 30 includes a transversal filter 32
serving as an adaptive phase control filter, a phase determination
unit 34 for detecting a phase space to which the demodulated symbol
data corresponds, on the basis of the output of the transversal
filter 32, a subtractor 36 for subtracting the output of the
transversal filter 32 from the output of the phase determination
unit 34, and an adaptive control algorithm processor 38 for
determining the coefficient of taps of the transversal filter 32 so
as to minimize the phase error data output from the subtractor 36.
The phase compensation control unit 30 forms a fractionally spaced
equalizer which needs no training signal when performing phase
compensation control.
[0029] In the present embodiment, the adaptive control algorithm
processor 38 employs an LMS (least means square) algorithm.
However, other adaptive control algorithms, such as an RLS
(recursive least square) algorithm, may also be employed.
[0030] The radio communication apparatus also includes a frequency
error evaluator 40 which acquires an operation result output from
the phase difference averaging unit 24 and determines whether the
frequency synchronization control performed by the frequency
synchronization control unit 20 has reduced the frequency error of
the reference signal output from the voltage controlled oscillator
12 relative to the received signal to a level within a
predetermined range which allows the phase compensation control
unit 30 to perform adaptive phase control. In this specific
embodiment, the predetermined range which allows the phase
compensation control unit 30 to perform adaptive phase control is
.pi./4 represented in phase for one cycle.
[0031] If the frequency error evaluator 40 has determined that the
frequency synchronization control performed by the frequency
synchronization control unit 20 has reduced the frequency error of
the reference signal output from the voltage controlled oscillator
12 relative to the received signal to a level within the
predetermined range which allows the phase compensation control
unit 30 to perform adaptive phase control, the phase compensation
control unit 30 performs adaptive phase control on the demodulated
signal output from the quadrature demodulator 18 in accordance with
the evaluation result output from the frequency error evaluator 40.
In this embodiment, a response speed of the frequency correction
control performed by the frequency synchronization control unit 20
is set to be slower than a response speed of the adaptive phase
control performed by the phase compensation control unit 30. This
allows a control loop formed by the frequency synchronization
control unit 20 to have a narrow band.
[0032] Referring to FIGS. 2 to 6, an operation of the radio
communication apparatus of the present embodiment will now be
described. FIGS. 2 and 3 illustrate the control process performed
by the frequency synchronization control unit 20, and FIG. 4
illustrates the control process performed by the phase compensation
control unit 30. If a signal is received via the antenna 10, the
multiplier 14 multiplies the received signal by the reference
carrier signal (with a frequency of fc) output by the voltage
controlled oscillator 12 thereby converting it to a baseband
signal. The baseband signal is then converted from an analog form
into a digital form by the analog-to-digital converter 16, and the
resultant signal is input to the quadrature demodulator 18. The
quadrature demodulator 18 converts, by quadrature demodulation, the
baseband signal in digital form into an in-phase component (I
component) and a quadrature component (Q component).
[0033] Subsequently, frequency synchronization control is performed
by the frequency synchronization control unit 20 as shown in FIGS.
2 and 3. That is, in step 100, the over-sampling phase difference
detector 22 performs over-sampling on the demodulated output x(i)
output from the quadrature demodulator 18 and detects phase
difference data at time intervals shorter than symbol intervals.
The demodulated signal x(i) can be represented as
x(i)=rk.multidot.exp{j(.phi.+2.pi..DELTA.fkT)} (1)
[0034] where rk is the amplitude, .phi. is the phase of the
modulation signal, .DELTA.fk is the frequency deviation, and T is
the sampling interval. The phase difference averaging unit 24 then
calculates the mean value .DELTA..theta.k of the phase difference
according to equation (2) described below. 1 k = 1 N n = 0 N - 1 (
a n / M + 2 f n T / M ) = 2 f T / M ( 2 )
[0035] where M is the over-sampling number, N is the number of data
used in the averaging calculation, and "an" is the transfer
function after the filtering.
[0036] In step 102, the over-sampling number M in equation (2) is
fixed to a particular value. After that, in accordance with
equation (2), the phase difference averaging unit 24 calculates the
phase difference mean value .DELTA..theta.k over a period of time
in which several symbols of the demodulated signal are output (step
104). The frequency deviation (i.e., the frequency error .DELTA.f
obtained in correspondence with the phase difference mean value
.DELTA..theta.k calculated in step 104) is then converted to a
voltage value by the frequency-to-voltage converter 26 and applied
to the voltage controlled oscillator 12 via the digital-to-analog
converter 28. As a result the frequency of the reference carrier
signal output from the voltage controlled oscillator 12 is
corrected such that the frequency error .DELTA. is reduced (step
106).
[0037] In the next step 108, the variance V of the mean phase
difference .DELTA..theta.k obtained in step 104 is calculated.
Then, it is determined in step 110, whether or not the variance V
is greater than a predetermined value j, thereby determining
whether or not the control loop formed by the frequency
synchronization control unit 20 is in a stable state. If it is
determined that V.gtoreq.j (i.e., if it is determined that the
control loop is in an unstable state), the number of data N is
incremented by +1 in the next step 112. After that, the process
returns to step 104, and steps 104 to 108 are repeated. As a result
of the increase in the number of data N, imbalance in occurrence of
data is leveled and noise is reduced. Thus, the control loop is
brought into a stable state from an unstable state due to an
imbalance in occurrence among data or due to a large phase
noise.
[0038] On the other hand, if it is determined in step 110 that
V<j, the process goes to step 114 to determine whether or not
V<k (where k<j). That is, it is determined whether or not the
control loop has come into a sufficiently stable state from the
above-described unstable state. Here, k is a value of the variance
V of the mean phase difference .DELTA..theta.k obtained when the
control loop is in the sufficiently stable state. When it is
determined that V.ltoreq.k in step 114, it is further determined in
step 115 whether or not N>.alpha. (where .alpha. is an arbitrary
integer). This judgement is necessary because the mean phase
difference .DELTA..theta.k will diverge if N becomes equal to 0 as
a result of the process performed in step 116. If N>.alpha., the
process goes to step 116. However, the process goes to step 104 if
N.ltoreq..alpha.. If it is determined in step 115 that
N>.alpha., the process goes to step 116, and the number of data
N is decremented by 1. After that, the process returns to step 104,
and steps 104 to 108 are repeated until the number of data N is
optimized. If the number of data N becomes large, data occurs in a
more random fashion (i.e., the occurrence probability becomes
similar for any data), and thus the frequency error becomes small.
However, the increase in the number of data N results in an
increase in the convergence time of the control system. Thus, it is
required to optimize the number of data N in the above-described
process.
[0039] On the other hand, if it is determined in step 114 that
k<V<j (i.e., if the control loop has become moderately stable
although not sufficiently stable), the process goes to step 118
shown in FIG. 3, and the number of data N in equation (2) is fixed
to a particular value. In the next step 120, the absolute value
.vertline..DELTA..theta.k.vertline. of the mean phase difference
.DELTA..theta.k is compared with a threshold value
.DELTA..theta.TH, where .DELTA..theta.TH>.pi./4M. If it is
determined in step 120 that
.vertline..DELTA..theta.k.vertline..gtoreq..D- ELTA..theta.TH, the
over-sampling number M in equation (2) is increased (step 122), and
the process then goes to step 124.
[0040] On the other hand, if it is determined in step 120 that
.vertline..DELTA..theta.k.vertline.<.DELTA..theta.TH, it is
further determined in step 127 whether or not the over-sampling
number M is greater than 2. If it is determined in step 127 that
M>2, the process goes to step 128, and the over-sampling number
M is reduced to M/2. After that, the process goes to step 130. On
the other hand, if it is determined in step 127 that M.ltoreq.2,
the process goes directly to step 130. In the above process, the
increase or decrease in the over-sampling number M depending on the
result of comparison of the absolute value
.vertline..DELTA..theta.k.vertline. of the mean phase difference
.DELTA..theta.k is required because the accuracy of detection of
the frequency error decreases with the reduction in the mean phase
difference .DELTA..theta.k or the frequency error .DELTA.f. That
is, the reduction in the accuracy of detection of the frequency
error is prevented by adjusting the value of the over-sampling
number M depending on the result of the calculation of equation
(2). Here, the increase/decrease in the over-sampling number M
corresponds to the increase/decrease in the sampling time. In the
first embodiment, as descried above, the accuracy of detection of
the frequency error is improved by adaptively controlling the
increase/decrease of the number of data (corresponding to the
sampling number) and the over-sampling number M (corresponding to
the sampling time) depending on the frequency error.
[0041] After the over-sampling number M in equation (2) is
increased in step 122, the mean phase difference .DELTA..theta.k is
calculated in step 124 in a similar manner as in step 104. In
addition, the frequency of the reference carrier signal generated
by the voltage controlled oscillator 12 is corrected in accordance
with the resultant mean phase difference .DELTA..theta.k in step
126 in a similar manner in step 106. In step 130, after step 126 or
step 128, the frequency error evaluator 40 determines whether
.vertline..DELTA..theta.k.vertline..ltoreq..pi./4. That is, the
frequency error evaluator 40 determines whether, as a result of the
frequency synchronization control performed by the frequency
synchronization control unit 20, the frequency error .DELTA.f or
the mean phase difference .vertline..DELTA..theta.k.vertline. has
become smaller than a predetermined value which allows the phase
compensation control mechanism 30 to properly perform adaptive
phase compensation.
[0042] If it is determined in step 130 that
.vertline..DELTA..theta.k.vert- line..gtoreq..pi./4, the process
returns to step 120, and steps 120 to 130 are repeated. On the
other hand, if it is determined in step 130 that
.vertline..DELTA..theta.k.vertline..ltoreq..pi./4, the process goes
to step 132 in which the phase compensation control unit 30
performs adaptive phase control in accordance with the result of
evaluation made by the frequency error evaluator 40. If it is
determined that the error of the frequency of the received signal
relative to the reference carrier signal generated by the voltage
controlled oscillator 12 has reduced by the frequency
synchronization control unit 20 to a level smaller than a
predetermined value (smaller than .pi./4 in phase), communication
to a sender may be started on the basis of that frequency. This
makes it possible for the sender to accomplish precise phase
compensation on a received signal.
[0043] The receiving operation may also be performed under
conditions in which the frequency error is determined to be within
the predetermined range by the frequency error evaluation unit 20.
When the receiving operation is again started after a pause, the
frequency synchronization control unit 20 performs frequency
synchronization control using the frequency of the reference signal
at that time such that the error of the frequency of a received
signal relative to the reference signal falls within the
predetermined range. In addition, when the receiving operation is
again started after another pause, the receiving operation is
performed using the same frequency of the reference signal as that
used in the previous receiving operation. This allows the reference
signal used for the frequency synchronization control to be updated
at shorter intervals. Thus, it becomes possible to achieve
frequency synchronization of the reference signal in a short time
even when the ambient temperature varies because of self heating or
a variation in environmental conditions.
[0044] The adaptive phase control performed by the phase
compensation control unit 30 is described below. In step 200 shown
in FIG. 4, the signal x(i) output from the quadrature demodulator
18 is input to the transversal filter 32 of the phase compensation
control unit 30. Phase compensation is performed on the demodulated
signal x(i) in accordance with the filter characteristic determined
by the filter coefficients set by the adaptive control algorithm
processor 38 (step 202). When the filter characteristic of the
transversal filter 32 is given by w(i), the output d(i) of the
transversal filter 32 is given as
d(i)=x(i).multidot.w(i) (3)
[0045] If the demodulated signal x(i) is given by
x(i)=exp{j(ak+2.pi..DELT- A.fT)}, the modulation phase component
and the phase error of the demodulated signal are given by exp(jak)
and e(i)=exp(j2.pi..DELTA.fT), respectively, and if the filter
characteristic of the transversal filter 32 is set as
w(i)=exp(-j2.pi..DELTA.fT) by the adaptive control algorithm
processor 38, the output d(i) of the transversal filter becomes 2 d
( i ) = x ( i ) w ( i ) = exp { j ( a k + 2 f T ) } exp ( - j2 f T
) = exp ( j a k ) ( 4 )
[0046] Thus, the phase error e(i) is completely eliminated, and the
original signal exp(jak) is extracted.
[0047] In the case where a 4-phase QPSK signal is demodulated, the
phase difference data .DELTA..theta.km (i.e., the difference in
phase between adjacent elements of the phase series {.theta.k}
output from the transversal filter 32) falls into one of four
quadrants of the I-Q coordinate system. That is, if the I axis is
taken as the reference, .DELTA..theta.k (k=1, . . . , 4)=.pi./4
(first quadrant), 3.pi./4 (second quadrant), -3.pi./4 (third
quadrant), or -.pi./4 (fourth quadrant). Thus, the symbol data
identified by the demodulated dibit becomes equal to one of
reference data D1, D2, D3, and D4 located in the respective
quadrants of the I-Q coordinate system.
[0048] In practice, however, the transversal filter cannot
completely eliminate the phase error, and the output d(i) of the
transversal filter 32 includes a residual error e(i). Thus, in step
204, the phase determination unit 34 determines in which quadrant,
in the orthogonal I-Q coordinate system, the phase difference
obtained from the output d(i) of the transversal filter 32 lies.
The phase determination unit 34 then determines that the input
symbol data corresponds to the reference data assigned to the
quadrant in which the input phase difference data .DELTA..theta.k
has been found to lie, and outputs the determined data (step 206).
Here, the output of the phase determination unit 34 is represented
by z(i).
[0049] In the next step 208, the subtractor 36 calculates the phase
error e(i) in accordance with equation (5) shown below.
e(i)=z(i)-d(i) (5)
[0050] The calculated phase error e(i) is input to the adaptive
control algorithm processor 38. The adaptive control algorithm
processor 38 processes the phase error e(i) in accordance with the
LMS algorithm so as to determine the filter coefficients of the
transversal filter 32 such that the phase error e(i) is minimized.
The transversal filter 32 is then set in accordance with the
resultant filter coefficients (steps 210 and 212).
[0051] Thus, as a result of the high-precision phase compensation,
the phase error e(i) falls within .pi./4 in absolute value as shown
in FIG. 6. FIG. 6 shows an example of the frequency error
characteristic. In FIG. 6, the horizontal axis represents the
frequency error .DELTA.fT, and the vertical axis represents the bit
error rate (BER). Curve C1 represents the frequency error
characteristic obtained after the phase compensation by the phase
compensation control mechanism 30, and curve Cn represents the
limit of the frequency error (the limit of the phase error)
acceptable by the phase compensation control mechanism 30 to
perform phase compensation. In FIG. 6, a frequency error .DELTA.fT
equal to 0.125 corresponds to a phase error of .pi./4.
[0052] In this first embodiment of the radio communication
apparatus which uses QPSK and in which frequency synchronization is
achieved on the basis of the phase difference data, at time
intervals shorter than symbol intervals, associated with a received
signal, the frequency of the reference signal used to achieve
frequency synchronization is corrected by the frequency
synchronization control mechanism in accordance with the mean value
of the phase difference data calculated over a period of time in
which two or more symbols are input. Thereby, it is ensured that
frequency synchronization can be achieved even when a phase
rotation greater than .pi./4 per cycle occurs, without using a
high-stability oscillator especially designed for frequency
synchronization.
[0053] In addition, in the first embodiment, the frequency of the
reference signal used to achieve frequency synchronization with the
received signal is corrected by the frequency synchronization
control mechanism in accordance with the mean value of the phase
difference data calculated over a period of time in which two or
more symbols are input such that the error of the frequency of the
reference signal relative to the frequency of the received signal
falls within the predetermined range acceptable to perform phase
compensation by the adaptive phase control. Also, the received and
demodulated signal is subjected to adaptive phase control performed
by the phase compensation control unit after frequency
synchronization has been achieved. This results in a reduction in
the frequency synchronization error due to a frequency offset.
[0054] Furthermore, the circuit loop for the frequency
synchronization is allowed to have a narrow band, and thus no phase
jitter occurs. As a result, the phase compensation control unit can
operate in a stable fashion. Thus, high-precision phase control is
achieved. That is, the reliability of the demodulated data is
improved.
[0055] Also, in the first embodiment, frequency synchronization is
accomplished by a combination of the correction of the frequency of
the reference signal by the frequency synchronization control unit
and the fractionally spaced equalizer serving as the phase
compensation unit, thereby ensuring that high-precision frequency
synchronization is achieved without needing a training signal.
[0056] Although in the first embodiment QPSK is employed as the
digital modulation method, other methods such as DPSK, QAM, etc.,
may also be employed to achieve similar features and advantages to
those obtained in the first embodiment of the invention.
[0057] FIG. 7 illustrates main parts of a second embodiment of a
radio communication apparatus according to the present invention.
This radio communication apparatus is different in construction
from that of the first embodiment shown in FIG. 1 in that a phase
error e(i) output from the subtractor 36 of the phase compensation
control unit 30 is added, via an adder 50, with the output of the
frequency-to-voltage converter 26 of the frequency synchronization
control unit 20, thereby correcting the output of the
frequency-to-voltage converter 26. The voltage controlled
oscillator 12 is controlled using the resultant corrected control
data as the control signal so as to control the frequency
synchronization. The other parts are similar to those of the first
embodiment, and thus they are not described in further detail.
[0058] In addition to the advantages and features of the first
embodiment, the radio communication apparatus according to the
second embodiment further has the advantage that phase compensation
for the frequency synchronization can be performed in a more
precise fashion than can be in the first embodiment.
[0059] Also, in the radio communication apparatus according to the
second embodiment, communication to a sender may be started after
the frequency of the reference carrier signal generated by the
voltage controlled oscillator is set in accordance with the
corrected control data described above. This makes it possible for
the sender to accomplish precise phase compensation on a received
signal.
[0060] Furthermore, in the radio communication apparatus according
to the second embodiment, the frequency of the reference carrier
signal generated by the voltage controlled oscillator which has
been set in accordance with the corrected control data described
above may be employed in a subsequent receiving operation. This
makes it possible to accomplish precise phase compensation in the
subsequent receiving operation.
[0061] Although QPSK is also employed as the digital modulation
method in the second embodiment as in the first embodiment, other
methods such as DPSK, QAM, etc., array also be employed to achieve
similar features and advantages to those obtained in the second
embodiment of the invention.
[0062] FIG. 8 illustrates an example of a radio communication
system according to the present invention. In FIG. 8, a service
area 300 includes a plurality of zones Z1, Z2, and Z3. In each
zone, communication between a base station and a mobile station is
performed using two frequencies one of which is used for upstream
transmission and the other is used for downstream transmission. The
respective base stations in each zone Z1, Z2, and Z3 use different
frequencies f1, f2, and f3 for communication. This radio
communication system employs a two-frequency simplex method in
which each base station in the zones Z1, Z2, and Z3 continuously
transmits a signal to a corresponding mobile station, whereas each
base station performs communication using a burst signal. In this
radio communication system, some of or all of the techniques
disclosed in the above embodiments of the invention may be
applied.
[0063] In addition, in this radio communication system, frequency
synchronization can be achieved even when a phase rotation greater
than .pi./4 per cycle occurs, without using a high-stability
oscillator specially designed for frequency synchronization.
[0064] In the radio communication apparatus which uses phase shift
keying (PSK), differential phase shift keying (DPSK), or quadrature
amplitude modulation (QAM), and in which frequency synchronization
is achieved on the basis of the phase difference data, at time
intervals shorter than symbol intervals, associated with a received
signal, and which includes frequency synchronization control unit
for correcting the frequency of the reference signal used to
achieve frequency synchronization, in accordance with the mean
value of the phase difference data calculated over a period of time
in which two or more symbols are input, a computer program for
implementing the functions of this radio communication apparatus
may be stored on a computer readable storage medium, and the
program may be loaded onto a computer system and executed thereby
accomplishing frequency synchronization control.
[0065] In this case, the program which implements the function of
the frequency synchronization is stored on the storage medium, and
the program is loaded onto the computer system and executed so that
frequency synchronization can be achieved even when a phase
rotation greater than .pi./4 per cycle occurs, without using a
high-stability oscillator specially designed for frequency
synchronization.
[0066] Furthermore, in the radio communication apparatus which uses
phase shift keying (PSK), differential phase shift keying (DPSK),
or quadrature amplitude modulation (QAM), and in which frequency
synchronization is achieved on the basis of the phase difference
data, at time intervals shorter than symbol intervals, associated
with a received signal, and which includes a frequency
synchronization control unit for correcting the frequency of a
reference signal used to achieve frequency synchronization with a
received signal, in accordance with the mean value of the phase
difference data calculated over a period of time in which two or
more symbols are input such that the error of the frequency of the
reference signal relative to the frequency of the received signal
falls within a predetermined range, and a phase compensation
control unit for adaptively controlling the phase of the received
signal when frequency synchronization has been achieved by
frequency control performed by the frequency synchronization unit
such that the error of the frequency of the reference signal
relative to the frequency of the received signal has fallen within
the predetermined range, a computer program for implementing the
functions of this radio communication apparatus may be stored on a
computer readable storage medium, and the program may be loaded
onto a computer system and executed thereby accomplishing frequency
synchronization control and phase compensation control.
[0067] In this case, the program which implements the control
function is stored on the storage medium, and the program is loaded
onto the computer system and executed thereby ensuring that the
frequency synchronization is accomplished with a less
synchronization error due to a frequency error.
[0068] In this case, the circuit loop for the frequency
synchronization is allowed to have a narrow band, and thus no phase
jitter occurs. As a result, the phase compensation control unit can
operate in a stable fashion, and thus high-precision phase control
is achieved. That is, the reliability of the demodulated data is
improved.
[0069] Furthermore, a computer program for implementing the
functions of the radio communication apparatus in which a
fractionally spaced equalizer is employed as the phase compensation
unit may be stored on a computer readable storage medium, and the
program may be loaded onto a computer system and executed thereby
accomplishing frequency synchronization control and phase
compensation control.
[0070] In this case, the program which implements the control
function is stored on the storage medium, and the program is loaded
onto the computer system and executed. This makes it possible to
accomplish frequency synchronization by a combination of the
correction of the frequency of the reference signal by the
frequency synchronization control unit and the fractionally spaced
equalizer serving as the phase compensation unit, thereby ensuring
that high-precision frequency synchronization is achieved without
needing a training signal.
[0071] Furthermore, a computer program for implementing the
functions of the radio communication apparatus, in which the
response speed of the frequency correction control performed y the
frequency synchronization control unit is set to be slower than the
response speed of the adaptive phase control performed by the phase
compensation control unit, may be stored on a computer readable
storage medium, and the program may be loaded onto a computer
system and executed thereby accomplishing frequency synchronization
control and phase compensation control.
[0072] In this case, the program which implements the control
function is stored on the storage medium, and the program is loaded
onto the computer system and executed. This allows the frequency
synchronization control unit forming a control loop for performing
frequency synchronization to have a narrow band.
[0073] Furthermore, a computer program for implementing the
functions of the radio communication apparatus, in which the
frequency synchronization control means adaptively increases or
decreases the sampling number and the sampling time of the phase
difference data subjected to the mean value calculation, in
accordance with the error of the frequency, may be stored on a
computer readable storage medium, and the program may be loaded
onto a computer system and executed thereby accomplishing frequency
synchronization control and phase compensation control.
[0074] In this case, the program which implements the control
function is stored on the storage medium, and the program is loaded
onto the computer system and executed. This makes it possible to
achieve an improvement in the detection accuracy of the frequency
error.
[0075] Furthermore, a computer readable storage medium may store a
computer program for implementing the functions of the radio
communication apparatus which includes a frequency error evaluation
unit for determining whether the error of the frequency of a
received signal relative to the frequency of the reference signal
has fallen within the predetermined range after the frequency
correction of the reference signal, and in which if the frequency
error evaluation unit has determined that the error of the
frequency is within the predetermined range, communication to a
sender is started on the basis of that frequency, and the program
may be loaded onto a computer system and executed thereby
controlling the communication process.
[0076] In this case, the program which implements the control
function is stored on the storage medium, and the program is loaded
onto the computer system and executed. This eases the frequency
synchronization in the sender apparatus.
[0077] Furthermore, a computer readable storage medium may store a
computer program for implementing the functions of the radio
communication apparatus in which control data used to control the
frequency of the reference signal and corresponding to the
frequency correction value required to achieve frequency
synchronization in the frequency correction control performed by
the frequency synchronization control unit is corrected by a
correction value corresponding to the value of phase compensation
made via adaptive phase control by the phase compensation control
unit, and the frequency of the reference signal is set in
accordance with the corrected control data, and then communication
to a sender is started, and the program may be loaded onto a
computer system and executed thereby controlling the communication
process.
[0078] In this case, the program which implements the control
function is stored on the storage medium, and the program is loaded
onto the computer system and executed thereby making it possible
for the sender to accomplish precise phase compensation on a
received signal.
[0079] Furthermore, a computer readable storage medium may store a
computer program for implementing the functions of the radio
communication apparatus in which a receiving operation is performed
under the conditions in which the frequency error is determined to
be within the predetermined range by the frequency error evaluation
unit; when the receiving operation is again started after a pause,
the frequency synchronization control unit performs frequency
synchronization control using the frequency of the reference signal
at that time such that the error of the frequency of a received
signal relative to the reference signal falls within the
predetermined range; and when the receiving operation is again
started after another pause, the receiving operation is performed
using the same frequency of the reference signal as that used in
the previous receiving operation, and the program may be loaded
onto a computer system and executed thereby controlling the
communication process.
[0080] In this case, the program which implements the control
function is stored on the storage medium, and the program is loaded
onto the computer system and executed thereby making it possible to
achieve frequency synchronization of the reference signal in a
short time even when the ambient temperature varies because of self
heating or a variation in environmental conditions.
[0081] Furthermore, a computer readable storage medium may store a
computer program for implementing the functions of the radio
communication apparatus in which the control data which is used to
control the frequency of the reference signal and which corresponds
to the frequency correction value required to achieve frequency
synchronization in the frequency correction control performed by
the frequency synchronization control unit is corrected by the
correction value corresponding to the value of phase compensation
made via adaptive phase control by the phase compensation control
unit, and the frequency of the reference signal set in accordance
with the corrected control data is employed in a subsequent
receiving operation, and the program may be loaded onto a computer
system and executed thereby controlling the communication
process.
[0082] In this case, the program which implements the control
function is stored on the storage medium, and the program is loaded
onto the computer system and executed thereby making it possible to
accomplish precise phase compensation in the subsequent receiving
operation.
[0083] Furthermore, a computer readable storage medium may store a
computer program for implementing the functions of the radio
communication system employing a two-frequency simplex method in
which communications between a base station and a plurality of
mobile stations are performed in such a manner that the base
station continuously transmits a signal whereas mobile stations
transmit a burst signal, in which the base station and the mobile
stations, or either the base station or the mobile stations, are
radio communication apparatus according to the present invention,
and the program may be loaded onto a computer system and executed
thereby controlling the communication process.
[0084] In this case, the program which implements the control
function is stored on the storage medium, and the program is loaded
onto the computer system and executed thereby ensuring that
frequency synchronization can be achieved even when a phase
rotation greater than .pi./4 per cycle occurs, without using a
high-stability oscillator specially designed for frequency
synchronization.
[0085] The "computer system" may include OS and hardware such as a
peripheral device. The "computer readable storage medium" is used
to refer to a wide variety of storage media. They include a
removable/portable medium such as a floppy disk, a magneto-optical
disk, a ROM, a CD-ROM, etc., and a storage device such as a hard
disk installed in a computer system, for example.
[0086] Furthermore, the "computer readable storage medium" also
includes a medium which dynamically stores a program for a short
time, such as an Internet network, a telephone line, and other
communication lines, via which a program is transmitted. In this
case, a storage medium such as a volatile memory which is installed
in a computer system serving as a server or a client and which
stores a program for a certain period of time is also a "computer
readable storage medium." The "program" may be a program which
implements some part of the functions described above. Furthermore,
the "program" may be such a program which is combined with a
program which has been already installed on a computer system to
implement the functions described above.
[0087] As described above, the present invention has various
advantages. That is, in the communication control method according
to the present invention, for the radio communication system which
uses phase shift keying (PSK), differential phase shift keying
(DPSK), or quadrature amplitude modulation (QAM) and in which
frequency synchronization between a mobile station and a base
station is achieved on the basis of the phase difference data, at
time intervals shorter than symbol intervals, associated with a
received signal, the frequency of the reference signal used to
achieve frequency synchronization is corrected in accordance with
the mean value of the phase difference data calculated over a
period of time in which two or more symbols are input, thereby
ensuring that frequency synchronization is achieved even when a
phase rotation greater than .pi./4 per cycle occurs., without using
a high-stability oscillator specially designed for frequency
synchronization.
[0088] Furthermore, the present invention provides the radio
communication apparatus which uses PSK, DPSK or QAM and in which
frequency synchronization is achieved on the basis of the phase
difference data, at time intervals shorter than symbol intervals,
associated with a received signal, in which the frequency of the
reference signal used to achieve frequency synchronization is
corrected by the frequency synchronization control unit in
accordance with the mean value of the phase difference data
calculated over a period, of time in which two or more symbols are
input, thereby ensuring that frequency synchronization is achieved
even when a phase rotation greater than .pi./4 per cycle occurs,
without using a high-stability oscillator specially designed for
frequency synchronization.
[0089] The present invention also provides the radio communication
apparatus which uses PSK, DPSK or QAM and in which frequency
synchronization is achieved on the basis of the phase difference
data, at time intervals shorter than symbol intervals, associated
with a received signal, wherein the frequency of the reference
signal used to achieve frequency synchronization with the received
signal is corrected by the frequency synchronization control unit
in accordance with the mean value of the phase difference data
calculated over a period of time in which two or more symbols are
input such that the error of the frequency of the reference signal
relative to the frequency of the received signal falls within the
predetermined range acceptable to perform phase compensation by the
adaptive phase control, and the received and demodulated signal is
subjected to adaptive phase control performed by the phase
compensation control unit after frequency synchronization has been
achieved, thereby ensuring that the frequency synchronization is
accomplished with a less synchronization error due to a frequency
error.
[0090] Furthermore, the circuit loop for the frequency
synchronization is allowed to have a narrow band, and thus no phase
jitter occurs. As a result, the phase compensation control means
can operate in a stable fashion, and thus high-precision phase
control is achieved. That is, the reliability of the demodulated
data is improved.
[0091] Preferably, the frequency synchronization is accomplished by
a combination of the correction of the frequency of the reference
signal by the frequency synchronization control unit and the
fractionally spaced equalizer serving as the phase compensation
unit, thereby ensuring that high-precision frequency
synchronization is achieved without needing a training signal.
[0092] Furthermore, the response speed of the frequency correction
control performed by the frequency synchronization control unit is
preferably set to be lower than the response speed of the adaptive
phase control performed by the phase compensation control unit.
This allows the frequency synchronization control unit in the
control loop for the frequency synchronization to have a narrow
band.
[0093] Preferably, the sampling number and the sampling time of the
phase difference data subjected to the mean value calculation are
adaptively increased or decreased by the frequency synchronization
control unit in accordance with the error of the frequency. This
allows an improvement in the detection accuracy of the frequency
error.
[0094] Preferably, when the frequency error evaluation unit has
determined that the error of the frequency of the reference signal
relative to the received signal has fallen within the predetermined
range after the frequency correction of the reference signal,
communication to a sender is started on the basis of that
frequency. This eases the frequency synchronization in the sender
apparatus.
[0095] Preferably, the control data used to control the frequency
of the reference signal and corresponding to the frequency
correction value required to achieve frequency synchronization in
the frequency correction control performed by the frequency
synchronization control unit is corrected by a correction value
corresponding to the value of phase compensation made via adaptive
phase control by the phase compensation control unit, and the
frequency of the reference signal is set in accordance with said
corrected control data, and then communication to a sender is
started, thereby making it possible for the sender to accomplish
precise phase compensation on a received signal.
[0096] The frequency synchronization control may be performed using
the same frequency of the reference signal as that used in the
previous receiving operation, thereby making it possible to achieve
frequency synchronization of the reference signal in a short time
even when the ambient temperature varies because of self heating or
a variation in environmental conditions.
[0097] Preferably, the control data which is used to control the
frequency of the reference signal and which corresponds to the
frequency correction value required to achieve frequency
synchronization in the frequency correction control performed by
the frequency synchronization control unit is corrected by the
correction value corresponding to the value of phase compensation
made via adaptive phase control by the phase compensation control
unit, and the frequency of the reference signal set in accordance
with the corrected control data is employed in a subsequent
receiving operation, thereby making it possible to accomplish
precise phase compensation in the subsequent receiving
operation.
[0098] Furthermore, in the radio communication system employing the
two-frequency simplex method, the base station and the plurality of
mobile stations, or either the base station or the mobile stations,
are formed of a radio communication apparatus according to the
present invention, thereby ensuring that frequency synchronization
can be achieved even when a phase rotation greater than .pi./4 per
cycle occurs, without using a high-stability oscillator specially
designed for frequency synchronization.
[0099] Furthermore, the present invention provides the
communication control method for the radio communication apparatus
which uses PSK, DPSK or QAM and in which frequency synchronization
is achieved on the basis of the phase difference data, at time
intervals shorter than symbol intervals, associated with a received
signal, in which frequency synchronization is controlled by
correcting the frequency of a reference signal used to achieve
frequency synchronization with a received signal, in accordance
with the mean value of the phase difference data calculated over a
period of time in which two or more symbols are input such that the
error of the frequency of the reference signal relative to the
frequency of the received signal falls within a predetermined
range, and the phase of the received signal is adaptively
controlled when frequency synchronization has been achieved, in the
step of controlling frequency synchronization, such that the error
of the frequency of the reference signal relative to the frequency
of the received signal has fallen within the predetermined range,
thereby ensuring that the frequency synchronization is accomplished
with a less synchronization error due to a frequency error.
[0100] In this case, the circuit loop for the frequency
synchronization is allowed to have a narrow band, and thus no phase
jitter occurs. As a result, the phase compensation control unit can
operate in a stable fashion, and thus high-precision phase control
is achieved. That is, the reliability of the demodulated data is
improved.
[0101] In addition, as previously discussed, the present invention
also provides a computer program product and corresponding computer
readable storage medium to achieve the advantages of the present
invention.
[0102] Obviously, numerous modifications and variations of the
present invention are possible in light of the above teachings. It
is therefore to be understood that within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described herein.
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